In the case of a fingerprint sensor, a raster-shaped arrangement of individual sensor elements that are assigned to the points of the image to be detected is usually present in one plane, and electronic circuit components for determining a signal are connected thereto. The individual sensor elements that are set up for capacitive measurement, for example, are driven in series. This can be done by row or column in the case of a rectangular raster and, thus, of a matrix-like arrangement of the pixels. To date, a fingerprint sensor is always started anew and delivers a complete fingerprint image after each start. This is done independently of how rich in contrast the image produced turns out to be. In particular, fingerprint images are also supplied whenever the finger is not yet correctly resting on the support surface of the sensor. It would be desirable, on the other hand, for it to be possible to operate the sensor in a fashion ensuring that complete fingerprint images are delivered only whenever a finger rests on the support surface such that a clear and contrasty image can be produced.
It is an object of the present invention to determine how a fingerprint sensor can be used to always produce sufficiently contrasty images when a finger is rested thereon.